Vapor generator



Filed 1. 1965 F. R. BELL ET AL VAPOR GENERATOR 5 Sheetg .s 1

llvvlwluus FRANCIS R. HORST F. MENZEL BELL July 22, 1969 R BELL ET AL 3,456,621

VAPOR GENERATOR Filed Oct. 1, 1965 5 Sheets-Sheet 2 I1\-'\ "/;'/\""l UR FRANCIS R. BELL HORST F. MENZEL F. R. BELL ET AL July 22, 1969 VAPOR GENERATOR 5 Sheets-Sheet 5 Filed Oct. 1, 1965 FIG. 4

IN Vli'N'l (1R5 FRANCIS R. BELL HORST F MENZEL July 22,1969 L ET AL 3,456,621

VAPOR GENERATOR Filed Oct. 1, 1965 5 Sheets-Sheet 4 FIG. 5

FRANCIS R BELL HORST F MENZEL zm M 5 F. R: BELL ET AL July 22, 1969 VAPOR GENERATOR 5 Sheets-Sheet 5 Filed Oct. 1 1965 BELL HORST F MENZEL FRANCIS FIG. IO

United States Patent 3,456,621 VAPOR GENERATOR Francis R. Bell, San Diego, and Horst F. Menzel, Poway,

Calif., assignor s, by mesne assignments, to Gulf General Atomic Incorporated, San Diego, Calif., a corporation of Delaware Filed Oct. 1, 1965, Ser. No. 491,955 Int. Cl. F22b 1/02, 1/14; F22g 1/02 US. Cl. 122-32 11 Claims ABSTRACT OF THE DISCLOSURE This invention relates to vapor generators such as are used in connection with the production of steam for driving steam turbines. More particularly, the invention relates to a steam generator which is especially suited for use with a gas cooled nuclear reactor in an electrical power generating facility.

Since the advent of nuclear power reactors, substantial steps have been taken toward the efficient and economical production of electrical power from thermal energy derived from these reactors. An important factor in the attainmment of this goal is the operation of such reactors at temperatures sufiiciently high to enable the direct production of steam at temperatures and pressures suitable for high efliciency operation of steam turbines. In this connection, present 'day reactor technology has led to the development of high temperature gas cooled reactors Which, when employed with a suitable steam turbine system, have the capability of producing electrical power of a quantity and at a cost which meet requirements of the utility industry.

In general, nuclear power plants employing high temperature gas cooled reactors enclose the reactor in a pressure vessel through which a fluid coolant, such as gaseous helium, is circulated to withdraw thermal energy liberated by the reactor. Steam for the operation of the turbines is normally obtained by the transfer of heat from the coolant to the fluid of a Water/steam system. Conventionally, such heat transfer is accomplished in a steam generator wherein the thermal energy withdrawn from the reactor is utilized to produce superheated steam.

In such a gas cooled reactor/ generator system, it is frequently desirable that the gas be caused to make only a single pass through the steam generator before being returned to the reactor. It is therefore important that the greatest possible amount of heat be withdrawn from the gas during the single pass to achieve maximum efliciency. It is also important, however, that there be as little restriction as possible to gas flow in order that work expended in transporting the gas through the system be held to a minimum. Where, for various reasons including structural economy, the steam generator is included in the same pressure bearing containment vessel as the reactor itself, it is also important that the size of the generator be minimized and that the steam generator or sections thereof be readily removable through necessarily restricted openings in the containment vessel.

It is an object of this invention to provide an improved vapor generator.

Another object of the invention is to provide an im- 3,456,621 Patented July 22, 1969 proved vapor generator capable of achieving an efiicient' transfer of heat between a working fluid and a gas making a single pass through the generator, and wherein restriction to gas flow is minimized.

Still another object of the invention is to provide an improved steam generator suitable for use in a high temperature gas cooled reactor wherein the steam generator is contained within the same pressure vessel as the reactor itself.

A further object of the invention is to provide an improved header arrangement for distributing steam to and collecting steam from a superheater-reheater section in a vapor generator.

Another object of the invention is to provide a vapor generator which is a self-contained unit in module form susceptible of incorporation in a system of substantially larger capacity, whereby the system may be constituted of a plurality of such vapor generator modules.

A still further object of the invention is to provide a novel arrangement of heat exchanging tubes for the purpose of accomplishing the foregoing objects.

Other objects and the various advantages of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawings wherein:

FIGURE 1 is a sectioned elevational view of a high temperature gas cooled reactor utilizing steam generators constructed in accordance with the invention, with the sectional planes passing through a helium circulator and through a steam generator to the center of the pressure vessel chamber;

FIGURE 2 is a sectional view taken along the line 22 of FIGURE 1;

FIGURE 3 is a sectional view taken along the line 33 of FIGURE 1;

FIGURE 4 is an enlarged full section elevational view of a part of a steam generator of FIGURE 1;

FIGURE 5 is a sectional view taken along the line 5-5 of FIGURE 4;

FIGURE 6 is an enlarged view of a segment of FIGURE 5;

FIGURE 7 is a sectional view taken along the line 7-1 of FIGURE 6;

FIGURE 8 is a schematic plan View illustrating the header and tube arrangement for the evaporator-econo: mizer section of the steam generator of FIGURE 4;

FIGURE 9 is a sectioned elevational view of a high temperature gas cooled reactor utilizing a steam generator constructed in accordance with an alternative embodi ment of the invention, with the sectional planes passing through a helium circulator and through a steam generator to the center of the pressure vessel chamber; and

FIGURE 10 is an enlarged full section view of a header utilized in the steam generator of FIGURE 9.

The vapor generator of the invention includes an evaporator-economizer section comprising a plurality of fluid conducting tubes. Each tube of the evaporator economizer section is formed in a frusto-conical helix and such tubes are nested adjacent each other along a common axis extending through each helix formed by the tubes. The vapor generator also includes a superheater and/or a reheater section comprising a plurality of fluid conducting tubes. Each of the tubes of the superheater or reheater section is formed in an elongated U shape having legs disposed parallel with the common axis. The tubes of the superheater or reheater section are arranged in groups in each of which the tubes thereof lie in the same plane. The groups are arranged in an annular series about the common axis with the plane of each group extending through the common axis. Means are provided in the vapor generator for directing a radial flow of preheated fiuid over the tubes of the evaporator-economizer section and the superheater or reheater section for transferring heat to the tubes and the fluid flowing in the tubes.

Referring now more particularly to the drawings, FIG- URES 1 through 3 illustrate a high temperature gas cooled nuclear reactor facility utilizing a steam generator constructed in accordance with the invention. The reactor includes a pressure bearing containment vessel comprised of a pre-stressed concrete structure 11 surrounding a chamber 12 provided with a reflective gas tight metallic liner 13. The liner 13 is made reflective to reduce heat transmission from the gas coolant. Cooling tubes 14 are placed in the concrete structure 11 and in contact with the liner 13 to remove the heat transmitted through the liner to the concrete. The concrete serves as a biological shield against radiation and is reinforced by pre-stressing tendons and conventional reinforcing steel in order to retain the integrity of the liner 13 under operating conditions. The exterior of the concrete structure 11 may be, for example, approximately 60 feet in height and 50 feet in diameter, while the interior or chamber 12 may be of the order of 35 feet in height and 25 feet in diameter.

The chamber 12 contains the reactor core 15 which is composed of vertical fuel elements 17 (only some of which are shown) approximately 20 feet long and arranged in the shape of a cylinder approximately 15 feet in diameter. Some of the fuel elements are shortened to accommodate ducts for the steam generators hereinafter described. An annular graphite reflector 19, which may be about three feet in thickness, surrounds the perimeter of the core 15. A steel core barrel 21, approximately 21 feet in diameter, surrounds the reflector 19 to contain the reflector and the core and to separate them from the annular space between the barrel 21 and the liner 13. A neutron shield 23 is mounted on the inside roof of the chamber 12 and a [plurality of penetrations 25 pass through the concrete structure 11 and the shield 23 for accommodating the control rod drives. A neutron shield 27 is disposed below the core and is supported by a core plate 29 reinforced by a plurality of transverse structural beams 31. The core plate 29 and hence the entire core structure is supported in the chamber 12 by an annular perforated ring 33 which bears on an annular shelf 35 extending around the periphery of the chamber 12 about V3 of the way up from the bottom thereof and secured to the liner 13.

Beneath the core barrel 21 is a large space about 10 feet high in which are disposed two steam generators 41, two helium circulators 45, and two emergency cooling systems 46 (see FIGURE 3). The steam generators 41 are located immediately beneath the reactor core 15 and associated supporting elements. A duct defined by wall 43 is provided above each steam generator 41 and is in fluid communication with the steam generator and the reactor core. An appropriate recess is provided in the reactor core for the duct.

In operation, the two helium circulators 45 force compressed and purified helium upwardly through a duct defined by wall 47 and through the perforated ring 33 into the annulus defined between the barrel 21 and liner 13. The helium circulators are driven by a shaft enclosed in a tube 48 extending through a passageway 44 in the pressure vessel 11. Some of the helium passes across the bottom of the core structure through openings provided in the supporting beams 31. The helium circulating upwardly in the annulus surrounding the core 15 blows across the top of the core and then down through the core between the fuel elements therein as indicated by the arrows in FIGURE 1. The fuel elements operate to raise the temperature of the helium from approximately 700 F. at the top of the core to approximately 1500 F. at the bottom.

At the bottom of the core, the hot helium passes through ducts 43 into one of the two steam generators 41. After passing through the steam generators 41, the hot helium moves back into the helium circulators 45 and restored to the appropriate pressure. The pressure at the outlet of the helium circulators 45 is preferably about 450 p.s.i.a., and the pressure drop through the steam generators is about 4 psi. where steam generators in accordance with the invention are utilized. As an alternative to the flow of helium shown in FIGURE 1, design modifications could be made such that the helium flow is upward along the reactor fuel elements and downward in the annulus surrounding the core 15. In either case, the flow of helium through the steam generators 41 is downward and radially outward as will be described in greater detail subseqently.

Referring now more particularly to FIGURES 4-8, a steam generator 41 is shown in greater detail. Steam generator 41 includes an evaporator-economizer section comprising a plurality of fluid conducting tubes 51. Each of the tubes 51 is formed in a frusto-conical helix. The outside of the steam generator 41 is surrounded by a cylindrical wall 53. Wall 53 is perforated at 54 to permit gas to flow therethrough while still providing structural support for the steam generator. As an alternative to the cylindrical wall 53, a plurality of vertical supporting rods could be utilized. A plurality of frusto-conical baflles 55 extend inwardly and upwardly from wall 53 and are supported thereby. The baffles 55 support the evpaoratoreconomizer tubes which are nested in bundles each disposed between a pair of the bafiles 55.

In each of the bundles of tubes of the evaporatoreconomizer section, there are ten tubes nesting in concentric stacked relation with the adjacent tubes staggered radially to permit close vertical spacing. Each tube makes sixteen loops in its inclined helical path and, accordingly, the cross section of each bundle appears as indicated in FIGURE 6. The bundles are arranged adjacent and one above the other in concentric stacked relation about a vertical common axis which coincides with the axis of each helix formed by the tubes in the bundles. In their nested arrangement, the bundles form a cylindrical structure about the axis.

Feed water is pumped into one end of each of the evaporator-economizer tubes 51 and flows through the tubes toward the other or discharge ends thereof. Heat transferred through the walls of the tubes to the water flowing therein from the gas flowing over the exterior surfaces of the tubes produces steam in the tubes which is drawn off at the discharge ends. Water is supplied to the evaporator-economizer tubes 51 by means of a supply header 57 communicating with the supply ends of the evaporator-economizer tubes (FIGURE 4). Feed water is supplied to the header through one of the fluid conduits 49 disposed in an opening in the bottom of the prestressed concrete pressure vessel 11.

A second header 61 (FIGURE 8), which is identical in construction to header 57, operates to receive the steam from the ends of evaporator-economizer tubes 5-1 and carry it exteriorly of the reactor chamber 12 through another one of fluid conduits 49. Header 57 is connected to the input ends of the various tubes 51 by means of a plurality of vertical fluid conducting tubes 65 arranged along the interior of the cylindrical wall 53 (FIGURE 8). Similarly, header 61 is connected to the output ends of tubes 51 by means of a plurality of vertical fluid conducting tubes 66 arranged along the inside of the tube coils. Appropriate horizontally disposed connections 67 extend from the peripheries of the headers to the various vertical tubes 65 and 66. 7

Feed water enters the coiled economizer-evaporator tubes 51 at the outer periphery of each of the bundles and follows upwardly an inclined circular path of decreasing radius toward the upper end of the tube, from where it is discharged as partially superheated steam. The steam attains full superheat in the separate U-tube superheater section subsequently described. The incline of the coiled tubes 51 is approximately 49 from the horizontal.

There are several advantages obtained from the arrangement of the evaporator-economizer tubes. First, the radial thickness of the annular space occupied by the tubes and support structure can be reduced significantly and, second, a turn of about 45 willimprove the helium flow pattern into the economizer-evaporator tube bundle and also reduce the turning losses at the tube bundle entrance and exit. In addition, the flow path from the water inlet to the steam outlet is slightly ascending, which enhances flow stability and drainability of the tubes.

Steam collected in header 61 is transferred by appropriate means, not shown, to the header of the superheater section of the steam generator wherein the steam attains the necessary degrees of sueprheat for efficient operation of steam turbines in the electrical power producing facility. The superheater section comprises a plurality of fluid conducting U-shaped tubes of an elongated configuration. Each tube has laterally spaced legs 69 which are parallel to the common axis of the evaporator-economizer bundles and, in the embodiment of FIGURES l to 8, extend along the cylindrical structure formed thereby adjacent the inner periphery thereof. A plurality of annular plates 71 extend horizontally and inwardly from the inner peripheries of the baflies 55 and are integral therewith to form continuous baffle surfaces. Plates 71 not only form gas directing baffles, but have a plurality of openings receiving the tubes 69 which are secured to and thus are supported by the plates 71.

The superheater tubes 69 are arranged in a plurality of groups, with the tubes in each group lying in a common plane extending through the axis for the evaporatoreconomizer tube bundles. The spacing of the legs of the tubes in each group serially decreases and each tube is disposed within the larger U of the immediately adjacent larger tube. In FIGURE 4, for purposes of clarity, only a few of the tubes 69 are shown. It is to be understood, however, that the various groups of U tubes are arranged in an annular series as shown in FIGURES 5 and 6 about the common axis. The respective planes of each group fan out around the common axis.

The lower ends of the tubes 69 are attached to respective sections of a double concentric header 73. Header 73 includes an upper chamber 75 connected to the lower ends of the inner legs of the U tubes and a lower chamber 77 to which the lower ends of the outer legs of the various U tubes are connected. The upper chamber 75 is for the input of steam to the tubes 69 from the evaporatoreconomizer section and appropriate connection is made to the latter section through a steam conducting tube 79. After attaining the desired superheat, the steam is removed through a passage 81 in fluid communication with the lower chamber 77. Passages 79 and 81 are formed by a plurality of concentric cylindrical walls 83 and 85 extending through an opening 86 in the lower portion of the concrete reactor structure 11.

It will be observed that the superheater section of U tubes 69 is concentric with and is telescoped within the cylinder defined by the helical tubes of the evaporatoreconomizer section to conserve the overall height of the steam generator. Hot helium heated by the fuel elements in the reactor flows downwardly into the steam generator and is directed outwardly by the baffles 53 and plates 71 through the perforations 54 in the cylindrical wall 53. In making its radially outward pass, the hot gas flows first over each of the tubes in the superheater section and then over the tubes in the evaporator-economizer section to effect the desired heat transfer in a single pass.

1 Referring now to FIGURES 9 and 10, a further embodiment of the invention is shown. In this embodiment, the steam generator comprises separate evaporatoreconomizer and superheater-reheater sections. Twosuch steam generators may be utilized, one on each side of a reactive core. The tubes of the reheater section are comingled with the superheater tubes. A reactor core is disposed in a chamber 112 in a pre-stressed concrete pressure vessel 111. Chamber 112 is lined by a metallic liner 113 which operates in the same fashion as the metallic liner 13 of the previous embodiment. A plurality of access openings are provided in the top of the core structure 111 for control rod drives and other access to the reactor core 115. The top of the reactor core 115 includes a control rod guide structure 116 in which are disposed a plurality of vents 118 for permitting the flow of gas from the reactor core into each of the two steam generators 141 located next to core 115. The reactor core 115 is supported on a steel support structure 131 which is perforated to permit upward flow of gas into and through the reactor core 115. The support structure 131 rests upon the bottom of chamber 112 by means of a surrounding cylindrical support 133. Alternatively, the support structure could rest on a pedestal extending upwardly from the floor of chamber 112. A pair of helium circulators 145 (only one of which is shown) are disposed in openings in the bottom of the concrete structure 111. Each circulator 145 includes an intake duct 147 through which helium is drawn from the annular space surrounding the reactor core after it has passed through the steam generators 141. Helium drawn through duct 147 is forced outwardly through a duct 143 lying beneath the perforated support structure 131. Helium then circulates upwardly through the reactor core 115, out openings 118 into the steam generators 141 and then into the annular space surrounding the core and back into the intake tube 147. Two helium circulators are provided and are of identical construction.

The construction of the evaporator-economizer section 140 of the steam generator 141 is identical to the construction of the evaporator-economizer section in the steam generator 41 of the previous embodiment. Further description of the evaporator-economizer section 140 of steam generator 141 is, therefore, deemed unnecessary.

The superheater-reheater section of the steam generator 141 comprises a plurality of elongated U tubes arranged in groups in which the tubes are in the same plane and are of different size, being disposed within each other. The groups of U tubes are arranged in an annular series about the common axis of the evaporator-economizer section and the plane of each group extends through the common axis. The tube arrangement is identical with that shown in connection with the previous embodiment except that alternate groups of the tubes are superheater and reheater tubes, respectively.

Connection is made to such tubes through a quadruple concentric header 171 which is shown in greater detail in FIGURE 10. The header easily accommodates a straight tube lead-in into the separate header chambers and also fits easily into a top opening 172 of the concrete containment vessel. The quadruple concentric header 171 comprises four cylindrical axially aligned adjacent chambers 173, 174, 175 and 176 which constitute input and output chambers for the superheater and reheater sections, respectively. Four passages are provided beneath the chambers and are defined by a plurality of concentric cylindrical walls 177, 178, 179 and 180. During the reheat portion of the steam cycle, the use of which is well known in the art, steam passes into the reheat input chamber 176 and from there into and through the reheat tubes and back out through the reheat output chamber 175. From there it may be recycled to lower pressure stages of the turbine.

U-shaped tubes. An appropriate gas conductor 182 links the superheater-reheater section with the evaporatoreconomizer section 140 disposed immediately below. The gas flowing into the evaporator-economizer section 140 is then directed radially outward by a plurality of frustoconical baffles 155 in an arrangement similar to that of the evaporator-economizer section of steam generator 41 of the previous embodiment. Helium is then collected in the annulus surrounding the reactor core 15 and drawn into the respective intake tubes 147 of the helium circulators 145 for recirculation through the reactor core.

One of the major advantages of both the steam generators 41 and 141 lies in the fact that these steam generators may be utilized in module form. With standardization of such individual modules, a complete range of power plant ratings can be covered by using such modules as integral building blocks. Furthermore, the module construction means that each module may be readily removed from the chambers in the pressurized pre-stressed concrete vessels through relatively small openings provided in the vessels for such purpose. This enhances the safety and constructional stability of the structures. In either of the two embodiments shown, a reheat cycle may or may not be used depending upon the particular construction and tube arrangement used.

Although the particular embodiments of the invention described herein are for the purposes of producing superheated steam from water, it is to be understood that the principles involved may be applicable to the vaporization of liquids other than water. The particular construction of the vapor generators of the invention permits a maximum heat transfer from the preheated fluid (helium in the described embodiments) to the fluid flowing in the tubes of the various sections while at the same time oifering a minimum of resistance to the flow of preheated fluid through the vapor generator. In the case of a gas cooled nuclear reactor, this aids in maintaining pressure in the gas flow system of the reactor. Other embodiments and modifications thereof will become apparent to those skilled in the art from the foregoing description and such are intended to fall within the scope of the appended claims.

We claim:

1. A vapor generator including in combination, an evaporator-economizer section comprising a plurality of fluid conducting first tubes, each of said first tubes being formed in a frusto-conical helix, said first tubes being nested adjacent each other along a common axis extending through each helix formed by said first tubes, a superheater section comprising a plurality of fluid conducting second tubes, each of said second tubes being formed in an elongated U shape having legs disposed generally parallel with said common axis, said second tubes being arranged annularly about said common axis within said evaporator-economizer section with the outermost legs of said second tubes being positioned adjacent the innermost loops of the helixes formed by said first tubes, and means for directing an outward radial flow of preheated fluid over said first and second tubes for transferring heat to fluid flowing in said first and second tubes.

2. The combination of claim 1 including means for directing the flow of preheated fluid downwardly into the space defined by said annular arrangement of said second tubes and then radially outward over said first and second tubes.

3. The combination of claim 1 wherein said second tubes are arranged in groups in each of which the tubes thereof lie generally in the same plane, said groups of said second tubes being arranged in an annular series about said common axis with the plane of each of said groups extending through said common axis, and wherein said second tubes in immediately adjacent groups are staggered radially relative to each other to permit close circumferential spacing thereof.

4. A vapor generator including in combination, an evaporator-eco'nomizer section comprising a plurality of fluid conducting first tubes, each of said first tubes being formed in a frusto-conical helix, said first tubes being nested adjacent each other along a common axis extending through each conical helix formed by said first tubes, a first header connected to one end of each of said first tubes for supplying evaporable fluid thereto, a second header connected to the other end of each of said first tubes for receiving vapor therefrom, a superheater section comprising a plurality of fluid conducting second tubes, a third header having a first vapor conducting passage therein connected to one end of each of said second tubes for supplying vapor thereto and having a second vapor conducting passage therein connected to the other end of each of said second tubes for receiving superheated vapor therefrom, each of said second tubes being formed in an elongated U-shape having legs disposed parallel with said common axis, said second tubes being arranged in groups in each of which the tubes thereof lie in the same plane, said groups of said second tubes being arranged in an annular series about said common axis with the plane of each of said groups extending through said common axis, and means for directing a radial flow of preheated fluid over said first and second tubes for transferring heat to fluid flowing in said first and second tubes.

5. The combination of claim 4 wherein said third header is aligned with said common axis and wherein said first and second headers are spaced from each other and said third header and are disposed directly beneath said first tubes.

6. A steam generator including in combination, an evaporator-economizer section comprising a plurality of fluid conducting first tubes, each of said first tubes being formed in a frusto-conical helix, said first tubes being arranged adjacent each other along a common axis extending through each helix formed by said first tubes, a first header connected to one end of each of said first tubes for supplying water thereto, a second header connected to the other end of each of said first tubes for receiving steam therefrom, a superheater-reheater section comprising a plurality of fluid conducting second tubes, some of which operate as superheating tubes and the remainder of which operate as reheating tubes, a third header for supplying steam to said second tubes and for receiving steam from said second tubes, said third header having first, second, third, and fourth axially aligned cylindrical chambers, said first chamber being connected to one end of each of said superheating tubes for supplying steam thereto, said second chamber being connected to the other end of each of said superheating tubes for receiving superheated steam therefrom, said third chamber being connected to one end of each of said reheating tubes for supplying steam thereto, said fourth chamber being connected to the other end of each of said reheating tubes for receiving reheated steam therefrom, said third header further having first, second, third and fourth steam conducting passages therein connected to said first, second, third and fourth chambers, respectively, said first, second, third and fourth steam conducting passages being defined by a plurality of concentric cylindrical walls, each of said second tubes being formed in an elongated U shape having legs disposed parallel with said common axis, said second tubes being arranged in groups in each of which the tubes thereof lie in the same plane, said groups of said second tubes being arranged in an annular series about said common axis with the plane of each of said groups extending through said common axis and with the superheating and reheating tubes being inter-mixed in their positions, and means for directing a radial flow of preheated fluid over said first and second tubes for transferring heat to fluid flowing in said first and second tubes.

7. In a vapor generator having a heat exchange section comprising a plurality of fluid conducting first tubes and a plurality of fluid conducting second tubes, a header for supplying steam to said tubes and for receiving steam from said tubes, said header comprising first, second, third and fourth axially aligned cylindrical chambers, said first chamber being connected to one end of each of said first tubes for supplying fluid thereto, said second chamber being connected to the other end of each of said first tubes for receiving fluid therefrom, said third chamber being connected to one end of each of said second tubes for supplying fluid thereto, said fourth chamber being connected to the other end of said second tubes for receiving fluid therefrom, said header further comprising first, second, third and fourth fluid conducting passages therein connected to said first, second, third and fourth chambers respectively, said first, second, third and fourth fluid conducting passages being defined by a plurality of concentric cylindrical walls in said header.

8. In a vapor generator, a heat exchanging section comprising a plurality of fluid conducting first tubes, each formed in a frusto-conical helix, said first tubes being nested adjacent each other along a common axis extending through each helix formed by the tubes, and a plurality of fluid conducting second tubes, each being formed in an elongated U shape with the legs thereof extending generally parallel with the common axis, said second tubes being arranged annularly about the common axis and being coaxial with said first tubes.

9. A vapor generator including in combination, a first heat exchanger section comprising a plurality of fluidconducting first tubes, each of said first tubes being formed in a frusto-conical helix, said first tubes being nested adjacent each other along a common axis extending through each helix formed by said first tubes, a second heat exchanger section disposed adjacent said first section and comprising a plurality of fluid-conducting second tubes,

each of said second tubes being formed in an elongated U shape having legs disposed generally parallel with said common axis, said second tubes being arranged annularly about said common axis, and means for directing a continuous radial flow of preheated fluid over said second tubes and then over said first tubes in a single pass for transferring heat to fluid flowing in said first and second tubes.

10. A vapor generator according to claim 9 wherein said nested first tubes and said annularly-arranged second tubes are coaxial on the common axis.

11. A vapor generator according to claim 9 wherein said nested first tubes and said annularly-arranged second tubes are axially aligned.

References Cited UNITED STATES PATENTS 2,160,644 5/1939 Clarkson 1222 O 2,672,849 3/1954 Fruit 122-333 2,982,266 5/1961 ROWand et a1. 122-478 3,052,222 9/ 1962 Fresch et a1 122-476 3,212,571 10/1965 Romanos et a1 163 3,279,449 10/ 1966 Arnrnon 122-34 KENNETH W. SPRAGUE, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,456,621 July 27 1969 Francis R. Bell et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 26, "evpaorat0r-" should read evaporator- Column 8,'line 69, "exchange" should read exchanger Column l0, line 24, "3,279,449" should read 3,279,439

Signed and sealed this 19th day of May 1970.

(SEAL) Attcst:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletehcr, Ir. Attesting Officer Commissioner of Patents 

